For a Macro-assisted system, conventionally, communication between a mobile station (MS) and a secondary base station (BS) is arranged by a link between the MS and a Macro BS. In this disclosure, MS is synonymous with user equipment (UE). In a Macro-assisted system, dual connectivity could be implemented in order to offload communications between a Macro BS and UEs to a secondary BS. Dual connectivity refers to a mobile station which simultaneously establishes two or more radio links to at least two base stations. FIG. 1 illustrates an example of a Macro-assisted system which adopts the above described dual connectivity scheme. In this example, a UE 103 would wirelessly establish a first link with the Macro BS 101 and wirelessly establish a second link with the secondary BS 102. The Macro BS 101 and the secondary BS 102 would establish communication via a backhaul link. In this way, data traffic between the Macro BS 101 and the MS 103 could be offloaded to the link between the MS 103 and the secondary BS 102 and also the backhaul link between the Macro BS 101 and the secondary BS 102.
The communication system in the future such as the upcoming 5G cellular communication system may gravitate toward a millimeter wave (mmWave) communication system which may operate above 30 GHz. Typically, an electromagnetic wave which operates above 30 GHz would experience severe pathloss and thus an antenna array that transmits a directional radiation pattern would be needed to implement directional transmissions. Similarly, a centimeter wave (cmWave) communication system may also operate within a directional transmission framework in the future.
For the above described directional communication system, dual connectivity could be implemented. For example, a Macro BS may entirely or partially implement omni-directional transmissions to accomplish configurations through control signaling in cmWave frequency or mmWave frequency; whereas, the secondary BS may implement directional transmission in mmWave frequency. FIG. 2 illustrates such an example as a Macro BS 201 has omni-directional transmission in cmWave frequency and directional transmission in mmWave frequency to cover a vehicle 202, a cell phone 203, and a D2D communication group 204 in which a secondary BS may or may not be used to implement directional transmission in mmWave frequency.
However, currently, assuming that there are multiple secondary BSs in the vicinity of a MS, the exact mechanism for a MS to search for an optimal secondary BS and to connect to the secondary BS is not yet known. FIG. 3 illustrates a hypothetical scenario in which a UE conducting a search among multiple candidate secondary BSs. In dual-connectivity system, a UE may search for one or more candidate secondary BSs for data transmission under the control signaling of a master (Macro) base station. Assuming that the UE has the capability for directional transmission and has at least four directions, which are 0, 90, 180, and 270 degrees, the best secondary BS which the UE may connect to would be based on different factors such as locations of the secondary BSs, which secondary BS has the best signal, the channel measurements, and etc. Also, what is considered the best secondary BS may change when the UE changes location, changes directional orientation, changes directional configuration, and other factors. Since currently the exact mechanism for a user device to discover the best secondary BS under the circumstance of connecting under a specific beam directional configuration is unknown, a method and related devices to discover a best secondary BS among a group of secondary BSs and subsequently to connect the best secondary BS could be needed.